Measurements of signal frequency based upon a cycle counter divide an integer number of input signal cycles by the time interval required to complete those cycles. Although the number of input cycles is exactly known, measuring the start and stop times of the interval itself includes errors from sources such as noise, non-linearity and quantization, which limit the precision of the results. In using a traditional reciprocal counter for frequency measurement, a time interval including precisely N signal cycles is begun and ended, and the start time is subtracted from the stop time to obtain the time interval length for frequency determination. These time interval start/stop measurements can be extended over many signal cycles in order to obtain a high precision average signal frequency value, which is allowed to vary with time. FIG. 1 illustrates this approach, which requires a relatively long time interval to obtain a single sample.
In another approach, sometimes called bicentroid measurement, a time interval is divided into three sub-intervals, each having approximately the same length. Event numbers, corresponding to events such as zero crossings of the signal, are measured for the first and third of these three intervals, with the second (intervening) time interval being used for instrument recovery or other purposes. The event numbers measured for the first and third of these intervals are then used to obtain an average frequency that represents the signal frequency in these time intervals, collectively. FIG. 2 illustrates the approach here.
One shortcoming of this approach is that the entire set of raw data or frequency measurements must be stored in memory, after which the microprocessor slowly post-processes the data one total time interval at a time. Another shortcoming is that, by the time the microprocessor has post-processed the data to obtain the frequency, time's moving hand has moved on and the frequency measurements are no longer current. Further, the individual frequency measurements are taken with respect to time intervals that are not contiguous to one another. Substantial, and possibly discontinuous, changes in frequency may occur in the intervening time intervals T.sub.2. Further, the measurement rate is very low because the length of the time gap between one pair of time intervals T.sub.1 /T.sub.3 and another similar pair is large compared to the nominal length of T.sub.1 or T.sub.3.
Several workers have disclosed other approaches to frequency measurement of undulating signals. Chu and Ward, in U.S. Pat. No. 4,519,091, disclose a non-interruptible counter for data capture in which an N-bit counter is arranged as an M-bit, fast, synchronous counter plus an (N-M)-bit, slower, ripple-through counter. The M-bit counter receives the M least significant bits, the (N-M)-bit counter receives the N-M most significant bits, and ripple through of a carry bit occurs slowly only in the latter counter. Each counter has an associated storage device, of size M bits and N-M bits, and entry of data into a storage device is time delayed by a predetermined amount to take account of non-zero bit settling time. First and second parallel counters may be used to count (1) the number of occurrences .DELTA.n of an event and (2) the length .DELTA.t of the measurement time interval during which these occurrences are sensed, within a predetermined time interval, and the frequency of occurrences within the measurement time interval is defined as .DELTA.n/.DELTA.t.
U.S. Pat. No. 4,541,105, issued to Lee et al., discloses receipt of a sample input signal, a reference input signal F.sub.R and a sensor input signal F.sub.s by a counter for frequency sampling purposes. The (low frequency) sample input signal defines a sequence of successive sampling intervals, within which the number of occurrences n.sub.s and n.sub.R of F.sub.s and F.sub.R, respectively, at a predetermined level are counted. The reference signal F.sub.R might be a sequence of clock pulses of frequency much higher than the nominal frequency of F.sub.s. The frequency of occurrence of the given level of F.sub.s is determined by reference to a ratio of n.sub.s and n.sub.R within each sample interval.
In U.S. Pat. No. 4,786,861, Hulsing and Lee disclose frequency counting apparatus with fractional count capability, using sampling time intervals and sensor signal occurrences that are determined in a manner similar to the occurrences in the Lee et al. patent discussed above. The number of signal cycles C.sub.n sensed in the nth time interval consists of an integer part and a fractional part and is defined by EQU C.sub.n =C.sub.n-1 -.DELTA.f.sub.n-1 +.DELTA.f.sub.n,
where f.sub.n is the fractional cycle associated with the nth sampling time interval.
Frederich, in U.S. Pat. No. 4,800,508, discloses frequency measurement apparatus in which the beginning of a sampling interval is synchronized with occurrence of an undulating signal pulse level that is to be counted. The end of a sampling interval appears to be determined by the last occurrence of the given level of the undulating signal within a nominal time interval.
What is needed here is a method of frequency measurement that provides controllably high temporal resolution and takes account of the changes with time of the signal frequency that is extant in a given short time interval. Preferably, the approach should allow frequency measurements with little or no "dead time", should require little or no memory for the raw data received, and should allow for prompt, post-processing so that an average frequency in a given time interval can be displayed shortly after the individual frequency measurements are made.